Different types of catchments are to be marked on the site plan. The collection efficiency of a particular catchment will be determined by the fact whether the catchment will be paved, unpaved or roof.

Area of the catchments

The amount of rainfall that will be collected will depend directly on the area of the catchment-the larger the area, the more the water. The area from where water would be collected will be arrived at by multiplying the length by the breadth of this space under the roof.

Location of the catchments

Paved areas (driveways/roads)

The quality of water that will be collected from the catchment will depend on the location of the catchment. Roof catchment provides the best quality of water. In areas where the catchments are open to contamination or are chemically treated then the water must be treated before being used for any purpose. Care must be taken when harvesting water from industrial areas.

The annual average rainfall: Will give an overall picture of the total amount of water that can be collected.

The pattern of rainfall over different months: Will tell you when the rainfall is available – is it available most of the year or only during a certain part of the year.

Number of rainy days: Will give an indication to decide whether to store the rainwater or to recharge it. If most of the rainfall comes only in a short span of time, then it is better to recharge the aquifer.

The peak rainfall intensity: Will give an indication to design the size of the storage or recharge structure. The sizing will be based on how much water will need to be stored or recharged during the most intense spell of rain.

C. Geological and hydrogeological data
For systems where the harvested rainwater will be used to recharge the aquifer, selection of site is important. Information must be collected on the following

The aquifer should be unconfined and must have good hydraulic conductivity as well as transmissivity so that the water that is recharged is quickly spread horizontally to prevent a water mound forming below the surface.

Depth of water table

Shallow or deep water table zones

The aquifer must not be at shallow depths and should be at least 8-10 metres below the ground level.

D. Water demand

Activity

Litres

Taking a bath from a bucket ( 1-2 buckets)

15-30

Shower (not power shower) for 15 minutes

60-80

Brushing teeth with the tap running

6

Brushing Teeth with the tap off

1

Washing utensils

30

Mopping

10

Drinking & cooking

6-10

Flushing toilet for one use (normal)

12-15

Flushingtoilet (low water use)

6

Washing Machine

100-150

Washing clothes without machine

30-45

Car Washing using a bucket

8

Car washing using a hosepipe

500

Gardening (2-4 buckets)depending on size may be more

30-60

Compiled from various sources

The size of the water harvesting structure is determined by two factors – how much is needed and how much is available.

Quantity of water currently used: This will give the total water demand and an indication of what portion of this total water need can be met from rainwater harvesting.

Per capita water demand: In case you cannot find out the exact amount of water used, you can find out the number of persons and multiply this with the per capita norm for water supply to arrive at the total water demand.

Water demand during the driest period: This is to help you estimate the most essential quantum of water needed during the driest period so that plans can be made for rainwater harvesting to meet this minimum need.

E. Legislation and incentivesToday many state governments and city municipalities have passed laws that make it mandatory for existing or new buildings to have rainwater harvesting systems. At the same time, there are also many incentives to motivate people to take up rainwater harvesting. You must find out about what incentives are available in your city.

Once all the relevant information has been collected, the next step is to study the site plan:

From the site plan find out the space available for water harvesting structures. This will determine the size and location of the structures.

Note the number and location of existing rain water pipes, outlets/spouts.

Find out if there are any defunct or existing borewells, swimming pool, water storage tanks that can be used for storing the harvested water. In a colony delineate all the open spaces from where water can be harvested as well as stored.

Determine the natural drainage, slope and location of storm water drains. This will help to lay out the conveyance pipes along the natural drainage patterns. This is particularly important while planning for a large complex or colony.

Mark the location of plumbing (water and sewage) and electrical lines in the site. Care must be taken to avoid plumbing and electrical lines while constructing the water harvesting structures. In case of project in public places this becomes even more important that underground sewer, water supply and other such cables and lines are not inadvertently destroyed.

Other information such as the existence and location of generator room, compost pit, waste dump etc also need to be taken into account.

The water harvesting structures should be as close as possible to the source and use of water.

Total volume of water = Area x runoff coefficient x rainfall

There is some loss of water due to evaporation or absorption by catchment surfaces and other kinds of losses. The runoff coefficent of a catchment gives you the proportion of the rainwater that can be harvested from the total rainfall.

Storage, recharge or both: The decision about whether to make storage or recharge structures depends on a number of factors as explained in table below:

Fractured, faulted or folded rocks, or comprises of weathered, jointed or fissured rocks

Recharge and storage

Nature of rainfall and monsoon

Number of rainy days are more, bimodal monsoon, not intensive, uniformly distributed

Storage

Unimodal monsoon, rainfall available only for a few months

Recharge and storage

Number of structures: The number of tanks will depend on the site conditions, which includes the position and location of the down pipes, the layout of the building, the size of the storage tank, the slope of the roof, the budget and the space available.

Capacity of storage tanks: Those will depend on the type of monsoon, bi-modal or uni-modal, number of rainy days, total demand and the rainfall intensity. If the rainy days are more, a smaller tank is sufficient as the tank can get frequently filled. The size will also depend on the demand and the total rainfall. Where the rainfall intensity is greater, the size will increase.

Location of structures: The location of structures will depend on the layout, the slope, the presence of other services and pipes and proximity to point of use.

Filtration and treatment: The type of filtration method used will depend on the use of the rainwater. If the harvested rainwater is going to be used for toilet or irrigation, then minimal filtration (with sand, gravel) is required to ensure that the water does not contain solid and toxic contaminants. On the other hand, when the water is to be used for drinking, then the level of treatment or filtration should be of high level.

The budget available for rainwater harvesting is one of the most important determinants of the type, size and number of water harvesting structures. In large projects, where there is a possibility of budgetary constraints it is better to design the project in phases. For more information on costs, click here